They seem to be doing OK. We won't know until the plants have finished flowering - another 5-6 weeks away - but with 400w running through each double-board frame (200w per QB324), they are easily keeping up with the 600w HPS plants next door.
The main reason I chose the QB324 boards was the spectrum: on spectrum alone, they beat the QB288/304 boards.
The Nichia LEDs in the QB324 are not as efficient as the LM301Bs, and you do not have the option of a CV driver or running in parallel, as the boards are about 108v each - 216v in series.
That said, the QB324s can be driven - in theory - twice as high as the QB288s. That is 300+W compared to 150W (QB324 uses 6.5v/150ma Nichia 757 series leds; QB288 uses 2.9v/180ma LM301B - although that max rating may have gone up to 3.1v/200ma, as I might be looking at an older spec sheet). HLG states maximum 250W for the QB324, which is really a limit of the board design, not the LEDs.
HLG also advises running the QB324s up to 160W with the Slate 2 heat sinks, but we have fans and aircon running in the room, so have pushed this to 200W without any apparent issues. At 200W per board, the LEDs are running at just over 60% of their max duty cycle, so this makes up somewhat for the reduced efficiency compared to the QB288/LM301Bs.
So, would I recommend them?
Yes and no. The combo boards with Slate 2 heatsinks are good value at their current price, and if you have active cooling, you can run them appreciably higher than the QB288/304 boards - so even better bang for buck.
The flowering spectrum is possibly the best on offer - though we will only know this for sure once we have finished flowering.
The caveat is, it also depends on your style of grow. Our plan is to use two Quantum Boards over 1 plant to replicate a 1 plant/600W HPS grow (we are trying to limit numbers for legal reasons).
However, if you scrog, then I like strips, as they offer better light distribution and heat dissipation. The new H inFlux Samsung strips look awesome - though they're not cheap. The F Series still offer great value. But by the time you have mounted them on aluminium U-channel etc, you may not be saving as much over the QB324 combo offer, and this also doesn't take into account the amount of time you spend sourcing and building the frames. The QB324s are easy to mount.
The only real killer for me is shipping cost: US$85 for four boards and heatsinks compared to free shipping from Arrow/Digikey. If you are in the US, it's a no-brainer - the QB324s are excellent value, IMO.
As for results, I'll let you know in a few weeks how we're doing. If it's successful, as I mentioned earlier, we may buy a bunch more of these boards - so that's probably your answer there!